Electronic power converters are currently used in a wide range of applications where a DC/AC conversion is required, carried out by means of inverters comprised by said converters, such as variable-speed drives, variable-speed wind turbines, solar inverters, UPS (Uninterruptible Power Supplies) systems or FACTS (Flexible AC Transmission Systems) devices.
Inverters in electronic power converters comprise static semiconductor-type switches. The switching characteristics of the semiconductor devices currently available on the market enable the most suitable semiconductor for each type of application to be chosen. As a result, depending on the power level required or demanded, different families of semiconductors may be identified:                MOSFETs: These are FET-technology semiconductors, ideal for low power/voltage and high-frequency-switching applications, such as switched sources and photovoltaic inverters. They are the most widely used in mass-produced consumer appliances.        IGBTs and IEGTs: Transistor-technology semiconductors. The IGBT has become the standard in low- and mid-power applications and in multi-MW applications with multi-level topologies. Mitsubishi has recently developed the IEGT with encapsulated press-pack for mid-voltage and high-power applications, as a result of which three-phase inverters of up to 10 MVAs can now be made. These are used in industrial drives, electric rail traction and equipment for renewable-energy generators (solar and wind), for example.        GTOs and IGCTs: Thyristor-technology semiconductors, provided with drivers that enable them to operate with forced switching. As with IEGTs, converters with power units of approximately 10MW can be developed, although the switching frequency is limited to frequencies in the region of 200 Hz for GTOs and 1000 Hz for IGCTs. They may be applied, for example, in high-power drives, FACTS devices, which may typically be SSSC (Static Series Synchronous Compensator) if connected in series with a transmission and/or distribution line, or STATCOM if connected in parallel with a transmission and/or distribution line, or UPFC (Universal Power Flow Controller), this being a combination of SSSC and STATCOM.        
The output voltage of the converter can be increased by increasing the number of levels of its output voltage, thereby increasing the power of the converter, which is achieved by using multi-level inverters. In addition, the quality of the waveform of the output voltage increases with the number of levels. As a result, with a three-level inverter, for example, it is possible to obtain a five-level output voltage waveform. The greater the number of levels, the greater the complexity in implementing the converter (of the inverters), as a result of which, and generally speaking, industrial applications are usually based on inverters or branches of up to two or three levels at most.
The most commonly used solution in the manufacture of high-power converters for FACTS applications, for example, is the connection of three-phase inverters of two or three levels to each other by means of intermediate magnetic elements or transformers, so that thanks to said connection or combination, another increase in the output voltage, and, therefore, in the power of the converter, is achieved, also improving the quality of the output waveform. For example, U.S. Pat. No. 3,628,123 A discloses the combination in parallel of two inverters by means of interphase transformers or IPTs.
U.S. Pat. No. 5,889,668 A discloses an electronic power converter. In said converter, a DC voltage is converted into an alternating voltage obtaining different alternating voltage waves by means of different inverters, a plurality of alternating output voltage waves being obtained. Said alternating output voltage waves are combined in parallel in twos, by means of interphase transformers or IPTs, until two resulting alternating voltage waves are obtained and which are handled for the elimination of at least some of their harmonics. The two resulting alternating voltage waves then reach two secondary windings of a coupling transformer by means of which the electronic power converter is coupled to the transmission and/or distribution line. To reduce the harmonic content of the voltage in the primary of the coupling transformer, the windings of one secondary are connected in wye and the windings of the other secondary in delta.